The present invention relates to devices for use in orthopaedic surgery, and especially for proper alignment of surgical instruments used in preparing a bone for an implant. The invention has particular application in preparing the distal end of the femur to receive a femoral prosthesis.
Damage or disease can deteriorate the bones, articular cartilage and ligaments of human joints, such as the knee, which can ultimately affect the ability of the natural joint to function properly. To address these conditions, prosthetic joints have been developed that are mounted to prepared ends of the bones of the joint, namely the tibia and femur in the case of a knee prosthesis. Among the many knee prostheses, a mobile bearing knee simulates the condylar and bearing surfaces of the knee to emulate the natural movement of the knee during flexion and extension. The tibial component is configured to permit rotation about the axis of the tibia to accurately replicate the effects of differential rollback in the transverse plane.
Implantable mobile bearing knee prostheses, such as the prosthesis 10 shown in FIG. 1, for diseased and/or damaged knees typically include three components, namely a tibial component 12, a femoral component 16 and a meniscal component (not shown). The tibial component 12 includes a platform 13 with a stem 14 configured for engagement in the prepared proximal end of the tibia. Generally, in a total knee joint replacement the platform 13 replaces the entire superior surface of the tibial plateau and substitutes for the tibial condylar surfaces. The femoral component can also include laterally-spaced condylar portions joined by an inter-condylar bridge and a patellar surface.
The femoral component 16 defines interior mounting surfaces 17 that often require involved cuts into the distal end of the femur. Since the components of the mobile bearing knee prosthesis 10 are generally configured to restore or emulate as much of the natural motion of the knee joint as possible, the femoral component often has a complicated geometry, which requires significant modification to the femur to accept and support the implant. The selection of the particular prosthesis components is usually dictated by the condition of the patient's knee. For instance, the condition of the distal end of the femur and proximal end of the tibia, as well as the patency of the surrounding ligaments and soft tissue can affect the form of the joint prosthesis.
In addition to the overall implant geometry, implant positioning with respect to the natural bone is critical. For instance, a proper implant will maintain the proper tension in the retained ligaments supporting the joint. In total knee reconstruction surgery, the menisci, bone ends and other stabilizing tissues are removed and replaced with implants. The thicknesses of the implants are ideally equal to the thickness of the removed material. Exceptions occur in reconstruction of severe deformity, where ligament length and tension after tissue releases during the reconstruction vary significantly form the preoperative state and from the normal knee.
Intraoperatively, the gap between the facing ends of the bones of the joint, which are related to the final implant position, can be manipulated. In the knee, a critical measure is the gap when the knee is in flexion or extension. The bone gaps in an ideal surgical reconstruction will have be the same in flexion and extension, the only exception being with implant systems having uneven implant thicknesses between anterior and posterior, or between medial and lateral compartments on either the tibial or femoral implants. The bone gaps for implants with unequal thicknesses must be accommodated for by the measuring tool or in the measurements when accessing potential implant fit. An ideal implant will maintain the same tension in flexion and extension, and the resulting joint tension and the stability of the implant will be substantially identical to the joint tension and stability of the patient's natural knee.
In preparing a knee joint, for instance, to receive a prosthesis, the orthopaedic surgeon typically uses templates to determine the proper size of the implant components. The surgeon may also measure the joint gap and choose a spacer that can be used in the procedure to maintain that gap. Since the femoral component of the knee prosthesis requires complex cuts in the femur, a femoral resection guide is used, such as the resection guide 20 shown in FIG. 2. The main body 22 of the guide 20 is aligned at the distal end of the femur F and held in place by one or more guide pins 24. The resection guide 20 may include other structure and components for maintaining the guide in a proper orientation as the femur is resected.
In order to ensure that the resulting femoral implant achieves the proper flexion and extension gaps, a femoral positioner 26 is often used. The femoral positioner shown in FIG. 2 includes a surface alignment plate 28 that rests on the previously resected surface R of the tibia. The alignment plate 28 is integral with a connector plate 30 that fits within a slot 23 in the main body 22 of the resection guide 20. The femoral positioner 26 is thus used to help position the resection guide so that the femur is properly resected.
Another known femoral resection guide 32 is depicted in FIG. 3. This guide includes a body 33 defining a slot 34 for receiving a saw. A stylus 36 is used to align the depth of the saw cut. Handles 40 can be provided to help stabilize the resection guide during a cut. Guide pins 38 extend into the femur F to align and support the resection guide.
It is important that the resection guide be properly oriented when the distal end of the femur is prepared, otherwise the femoral implant will be produce undue strain or laxity in the knee joint. It is critical to maintain equal flexion and extension gaps to restore the proper anatomic tension as much as possible, regardless of the nature of the knee prosthesis. For instance, most mobile bearing knees are modular, meaning that several bearing elements can be provided depending upon the patient's anatomy. Obviously, thicker bearing elements correspond to greater flexion/extension gaps.
Similar modularity is important in the guide instruments used to ensure proper manipulation of the bones of the joint. There is a need, therefor, for an augment that can be readily used in the orthopaedic procedure to allow the guide instruments to properly emulate the natural anatomy of the instrumented joint.
Moreover, there is a need for an augment that can account for variations in the quality of the underlying bone. This need is particularly acute for revision surgeries in which the bone may have defects that make finding a stable platform difficult.